Solid-bowl centrifuge decanters are well known. Typically such apparatus comprises an elongated bowl mounted for rotation about its longitudinal axis with a helical screw conveyor, coaxially mounted within the bowl, adapted to rotate at a speed slightly different than the speed of rotation of the bowl; the bowl is tapered or trunco-conical near its solids discharge end. The screw conveyor is formed of one or more helically arrange blades, which sweep the surface of the bowl of the apparatus while propelling the centrifugally separated solids toward the solids discharge port. An example of such equipment in which the blade is helically mounted on a hub is described in U.S. Pat. No. 3,764,062, the disclosure of which is incorporated herein by reference.
In operation of a solid-bowl centrifuge decanter with a screw conveyor, a solids-liquid feed is introduced into the bowl, where, due to centrifugal force effected by rotation of the bowl, the feed separates into its component parts with the heavier part, typically solids, being moved outboard of the other feed components in a pool of liquid, and adjacent to the inner surface of the bowl. Since the bowl and screw conveyor are rotated at predetermined different speeds, solids sedimented against the inner surface of the bowl are conveyed by the distal edge of the conveyor's blade along the bowl surface until separated from the pool of liquid and discharged from one or more ports at the tapered end of the bowl.
Commercially important uses of solid-bowl centrifuge decanters include separation of solid crystalline chemical compounds from liquids under process conditions which do not degrade quality, such as chemical purity of a desired crystalline product. Crystallization, as a commercial process, is significant because of the great variety of materials that are marketed in the crystalline form. Its wide use is due basically to the fact that a crystal forming from an impure solution is, generally, itself pure. Thus, crystallization affords a practical method of obtaining concentrated chemical substances in a form both pure and attractive and in suitable condition for packaging, handling, and storing.
Although a crystal itself is necessarily pure, it retains mother liquor when removed from the final solids-liquid mixture or magma, and the adhering mother liquor will carry its share of the impurities present in the mother liquor. If the retained mother liquor is dried on the crystal, product contamination results. In practice, centrifuging leaves mother liquor mounting as much as 50 percent of the weight of the crystals. Large uniform crystals from low-viscosity mother liquors will retain a minimum proportion of mother liquor, while non-uniform small crystals from viscous solutions will retain a considerably larger proportion.
Residual impurities in the centrifuge cake can be reduced by admixing a diluent liquid or solvent liquid with the solids-liquid centrifuge feed. Such diluent liquid lowers concentration of impurities in the mother liquor and consequently reduces the amount of residual impurities in the centrifuge cake even without a reduction in the amount of mother liquor retained. This method is highly inefficient and, particularly where compounds are crystallized from an organic solvent, it has additional capital and operating costs, because it requires using a very large amount of diluent per unit of solid product to achieve a significant reduction of residual impurities.
It is common practice to wash the crystals with fresh solvent on the centrifuge or in an additional slurry/separation unit. In principle such washing can reduce impurities to below almost any arbitrary level, but these methods are not usually so satisfactory as expected and may also have significant additional capital and operating costs.
For example, an evaluation of a continuous solid bowl centrifuge for processing simulated Rover dissolver effluent by J. S. Vavruska and J. A. Rindfleisch is described in Nucl. Sci. Abstr. 1975, 32(10), Abstr. No. 24218 (1975). A plant-scale continuous solid bowl centrifuge was used as a method of solids-liquid separation and solids washing. The solid bowl centrifuge reduced undissolved solids content of the liquid effluent of the Rover dissolver under conditions studied. Although wash injection nozzles were located within the centrifuge bowl, it is reported that little or no washing of occluded uranium from the solids was possible. The most effective removal of occluded uranium resulted from repulping undissolved solids in water in two or more stages and recycling the repulped slurry through the test centrifuge.
U.S. Pat. No. 3,971,509 to Frank Birger Johnsen describes a centrifuge comprising an outer drum and an inner rotor which carries at least two screw helixes for transporting solid matter axially towards discharge ports. An aperture is provided in each helix at its distal edge or outer periphery whereby patentee states that the axial movement of at least some of the transported solid matter is temporarily stopped when that matter passes through the aperture. A washing liquid is supplied through openings or spray nozzles in the rotor shortly upstream of each aperture. It is stated that a spray of washing liquid from the rotor can act upon the solid matter during the period in which its axial movement may be interrupted. When the screw is provided with an aperture in each of its helices, the interior of the rotor may be divided into at least two axially spaced chambers and a separate washing liquid used in connection with each chamber. Patentee states that it is possible to effect control of static pressure which acts upon the washing liquid in each of the chambers and determines the outflow velocity of the liquid spray from the chamber.
In washing cake on a conventional centrifuge, washing liquid flows over or is sprayed on top of sedimented cake near the point along the axis of the centrifuge where the cake is conveyed out of the liquid pool in a "beach" section of the centrifuge. For example, see U.S. Pat. No. 3,302,873 to Yasuo Kowata, U.S. Pat. No. 4,496,340 to Detmar Redeker and Dieter Mrotzek, or U.S. Pat. No. 4,654,022 to Leonard Shapiro. Spray washing in a conventional centrifuge is, however, not as effective as required because much of the washing liquid flows over the surface of the cake and into the liquid pool of mother liquor thus leaving the centrifuge without penetrating the bulk of the cake to displace mother liquor retained within the cake except on or near the surface of the cake.
U.S. Pat. No. 4,654,022 to Leonard Shapiro describes a centrifuge fabricated with a hollow flight member at the bowl dry beach area by welding sheet metal to trailing surfaces of the flight. Leading or working surface of the flight is provided with a plurality of series of orifices per 360.degree. revolution thereof, each series comprising spaced small orifices, typically 0.030 to 0.125 inches in diameter, spiraling inwardly from the outer edge of the flight and ending with large overflow orifices in the flight near its edge attached to the conveyor hub. In operation, rinse liquid is said to pass onto solids pile surfaces through orifices immediately inwardly of the pile surface. It is also stated that it is possible for small amounts of rinse liquid to pass through orifices submerged by the wedge of solids formed against flight adjacent to the bowl wall. Since the density of the solids pile becomes progressively greater as the bowl wall is approached in the dry beach area, Shapiro speculates that in this apparatus the amount of rinse liquid urged out of the submerged orifices is proportionately decreased as the orifices are disposed more outwardly in the flight. Most of the washing liquid is believed to flow over the surface of the cake and into the liquid pool thus diluting the mother liquor, but leaving the centrifuge without penetrating the bulk of the cake to displace mother liquor retained within the cake.
There remains, therefore, a current need for centrifugal apparatus which provides means for more effective washing of crystals with fresh solvent on the centrifuge. Advantageously, such means for washing of crystals in a centrifuge would require only a limited amount of washing liquid to penetrate a sedimented cake and effectively displace mother liquor from within the cake.